tools: git hooks: drop "submitted by" from commit template

With the switch to git, we should strive to properly attribute every
commit appropriately with the metadata that's provided to do so. In this
case, the submitter should be recorded via the author metadata.  Committing
an arbitrary patch, one can set it as such:

git commit --author="John Smith <smith@example.com>"

Reviewed-by:	emaste
Differential-Revision:	https://reviews.freebsd.org/D28069

tools/tools/git/hooks/prepare-commit-msg

@@ -43,7 +43,6 @@ $(awk '1;/^#$/{exit}' $1)
 # Uncomment and complete these metadata fields, as appropriate:
 # 
 # PR:		<If and which Problem Report is related.>
-# Submitted by:	<If someone else sent in the change.>
 # Reported by:	<If someone else reported the issue.>
 # Reviewed by:	<If someone else reviewed your modification.>
 # Approved by:	<If you needed approval for this commit.>
@@ -57,7 +56,8 @@ $(awk '1;/^#$/{exit}' $1)
 # Differential Revision:	<https://reviews.freebsd.org/D###>
 #
 # "Pull Request" and "Differential Revision" require the *full* GitHub or
-# Phabricator URL.
+# Phabricator URL.  The commit author should be set appropriately, using
+# \`git commit --author\` if someone besides the committer sent in the change.
 $(awk '/^#$/,EOF' "$1")
 EOF